Amid shifting opinions, intensification of the fight against ISIS appears increasingly necessary–and likely. Just how should that approach be shaped?

The first step is to understand the president’s discretion and powers in matters of national security. The Constitution and traditional deference to the executive in wartime put the scale and scope of the fight against ISIS almost exclusively in the hands of the president, top administration officials, and military commanders. Recall that in 2007, a majority in both houses of Congress called for the withdrawal of U.S. troops from Iraq, and polls showed a majority of Americans favored a withdrawal timetable–yet President George W. Bush increased U.S. troop levels. And while constraining a president’s national security decisions is hard, pushing him to intensify an effort against his inclination is more difficult. In 2012, key advisers to President Barack Obama, including his secretary of state and CIA director, urged him to arm Syrian rebels. He demurred.

In all this, public sentiments may prove a fickle guide. Actions that were unthinkable on Sept. 10, 2001–invading Afghanistan, toppling the Taliban, arming Predator drones–were seen as prudent and even necessary just days later. Today we see policymakers reflecting less on what the public will tolerate and focusing more on what the people will demand.

The president and his team would do well to flip these considerations on their head. Rather than reading public sentiment and reflecting those preferences in rhetoric and policy–as President Obama has done by repeatedly telling war-weary Americans that he would not return combat troops to Iraq nor put “boots on the ground” in Syria–they should let the policy drive the politics. This means making hard calls about dollars spent, troops deployed, allies cajoled, energy expended, and diplomacy pursued–and then articulating their reasons.

A good place to start would be to cut through the rhetorical cul de sac the president has worked himself into on troops in Syria and Iraq. Administration officials have argued feebly that the deployment of Special Operations forces to Syria doesn’t constitute “boots on the ground” and that the 3,000 U.S. troops in Iraq and the warplanes bombing both countries do not represent “combat.” Congress and the public would be far more likely to understand if they articulate the scale of the threat posed by Islamic State; explain why, notwithstanding Mr. Obama’s past commitments to avoid conflicts, the United States must combat that threat; and detail a sound concept for success.